I'd start with the benchmark tool that comes with postgresql: pgbench.

pgbench can be used immediately with several pre-set scenario's (e.g. write-heavy, readonly), but it can also be given custom SQL to test your own data/queries.

Or perhaps start right away with Greg Smith's pgbench-tools, which is built on top of pgbench. It produces output in useful series of text, graphs, html. Very nice.

I've heard good things about Tsung but never used it. It would seem that the pgbench-tools are the obvious first thing to run as they require almost no work to get going, and will provide you with a good first idea about your system's performance.

Lots of tuning information in the mailing lists; especially pgsql-performance (and perhaps, pgsql-hackers). The search interface on the postgresql website isn't very good but the mbox files are available too.

edit: Of course, quite possibly you have to use a much larger initialisation-value (the 'scale', -s, in the init invocation); and the performance obviously gets *much* worse when the tables do not fit in memory anymore or write-mostly is used as opposed to readonly, as above.